Program to find shortest subarray to be removed to make array sorted in Python

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Suppose we have an array called arr, we have to remove a subarray of arr such that the remaining elements in arr are in non-decreasing order. We have to find the length of the shortest subarray to remove.

So, if the input is like arr = [10,20,30,100,40,20,30,50], then the output will be 3 because we can remove [100, 40, 20] which is smallest subarray of length 3, and by removing these all are in non-decreasing order [10,20,30,30,50].

To solve this, we will follow these steps:

• n := size of arr
• arr := insert 0 at the left of arr and infinity at the right of arr
• A,B := two new empty lists
• p := 1, q:= size of arr -2
• M := 0
• while p <= q, do
• if arr[p-1] <= arr[p], then
• insert arr[p] at the end of A
• p := p + 1
• otherwise when arr[q] <= arr[q+1], then
• insert arr[q] at the end of B
• while A is not empty and last element of A > last element of B, do
• delete last element from A
• q := q - 1
• otherwise,
• come out from loop
• M := maximum of M and size of A + size of B
• return n - M

Let us see the following implementation to get better understanding:

Example

def solve(arr):
n = len(arr)
arr = [0] + arr + [float("inf")]
A,B=[],[]
p,q=1,len(arr)-2
M = 0
while p <= q:
if arr[p-1] <= arr[p]:
A.append(arr[p])
p += 1
elif arr[q] <= arr[q+1]:
B.append(arr[q])
while A and A[-1] > B[-1]:
A.pop()
q -= 1
else:
break
M = max(M, len(A)+len(B))
return n - M
arr = [10,20,30,100,40,20,30,50]
print(solve(arr))

Input

[10,20,30,100,40,20,30,50]


Output

3
Updated on 04-Oct-2021 08:12:57